std.complex: Add member function to returnComplex as a native type

[ibuclaw]

So that std.complex can be passed to extern(C) complex functions where ABI compatibility is required.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

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dub run digger -- build "master + phobos#8415"

[dukc]

As Dennis mentioned, this should have documentation. Maybe a documented unit test. The one you added would serve well IMO.

Otherwise LGTM.

[RazvanN7]

I took the liberty of rebasing this, adding a comment to the function and solving the buildkite failure.

@ibuclaw @dkorpel could please take a look?

[dukc]

Why not a Ddoc comment though?

[RazvanN7]

@dukc I simply followed the style that is currently present. But I made it a ddoc comment now.